The PCB is compatible with the EnvSensor firmware, so you can connect all the sensors like temp sensor / humidity sensor, light sensor etc. Even the "feature connector" is the same as on the Generic Midi. I2C is also supported.

A big advantage is that you can solder a CR 2032 battery on the back side. This makes the PCB incl. battery much smaller than a generic midi with AAA batteries. The capacity is only ~15% of 2 AAA batteries, but it's enough for e.g. window sensors. If you need more power, you can connect normal AAA or AA batteries in an (external) battery holder of course.

The PCB and a Kit is also available in the shop. Note that you should have experience in SMD soldering to build this device up, because especially the pin pitch of the ATMega is very small.